1. Field of the Invention
The present invention relates to a method of laser crystallization (LC), and more particularly, to a method of two-step laser crystallization having an enlarged process window.
2. Description of the Prior Art
Nowadays, a liquid crystal display(LCD)is the most mature flat panel display technique. The applications for a liquid crystal display are extensive, such as mobile phones, digital cameras, video cameras, notebooks, and monitors. Due to the high quality vision requirements and the expansion into new application fields, the LCD has developed toward high quality, high resolution, high brightness, and low price. A low temperature polysilicon thin film transistor (LTPS TFT), having a character of being actively driven, is a break-through in achieving the above objectives. Furthermore, a metal-oxide-semiconductor and the low temperature polysilicon thin film transistor in this technique are integrated in a same manufacturing process to fabricate a system on panel (SOP). The low temperature polysilicon thin film transistor therefore has become a booming development area for all vendors.
During the manufacturing process of the low temperature polysilicon thin film transistor liquid crystal display, a glass substrate tends to deform if the polysilicon film is directly formed at a high temperature since the resistance of the glass substrate to heat is merely up to 600° C. As a result, expensive quartz is utilized as the substrate for the traditional polysilicon thin film transistor liquid crystal display. The application is therefore limited to small sized liquid crystal display panels. Nowadays, a method to re-crystallize the amorphous silicon thin film has become popular and mainstream. More particularly, the excimer laser annealing (ELA) process is most significant.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a method of forming a polysilicon thin film by utilizing an excimer laser annealing process. As shown in FIG. 1, an amorphous silicon thin film 12 having a thickness of approximately 500 Å is deposited on a glass substrate 10 first. Then the glass substrate 10 is disposed in a hermetic chamber (not shown) to perform the excimer laser annealing process. The method for depositing the amorphous silicon thin film 12 comprises a low-pressure chemical vapor deposition (LPCVD) process, a plasma enhanced chemical vapor deposition (PECVD) process, or a sputtering process. When performing the excimer laser annealing process, the amorphous silicon thin film 12 on the surface of the glass substrate 10 is irradiated by the laser pulse 14 of the excimer laser through a transparent window (not shown) on the upper surface of the chamber (not shown). The laser pulse 14 scans the regions within a process scope, which is determined previously, step-by-step to heat the amorphous silicon thin film 12 within the process scope rapidly. The amorphous silicon thin film 12 is therefore re-crystallized into a polysilicon thin film (not shown).
Moreover, the amorphous silicon thin film is melted and re-crystallized rapidly through absorption of the deep ultraviolet light during the excimer laser annealing process to form the polysilicon thin film. Such a quick absorption due to the short laser pulse only affects the surface of the amorphous silicon thin film and will not affect the glass substrate. Hence, the glass substrate is kept in a low temperature state. The excimer lasers frequently used comprise a XeCl laser, an ArF laser, a KrF laser, and a XeF laser. Since the different molecules will generate light with different wavelengths, the output energy density is therefore adjusted according to the thickness of the amorphous silicon thin film. For example, the output energy density is approximately 200 to 400 mJ/cm2 for an amorphous silicon thin film with a thickness of 500 Å. After performing the excimer laser annealing process, the subsequent processes for forming the liquid crystal display panel are performed. The polysilicon thin film is used as a channel or a source/drain to form the driving circuit or the logic circuit on the liquid crystal display panel.
Since the quality of the amorphous silicon thin film 12 is a determinative factor for the characteristics of the subsequently formed polysilicon thin film, all of the parameters during the amorphous silicon thin film deposition process need to be strictly controlled. The amorphous silicon thin film with low hydrogen content, high thickness uniformity and low surface roughness is thus formed. In addition, many variables during the crystallization process will directly affect the grain size and the grain distribution after the crystallization process is completed. When non-uniform phenomenon occurs during the crystallization process, various types of defects emerge.
Please refer to FIG. 2. FIG. 2 is a schematic diagram of an energy density utilized in the prior art laser crystallization process. As shown in FIG. 2, the energy density E adapted in the prior art laser crystallization process is between a nearly-completely-melted energy density (ENCM) and a super lateral growth energy density (SLG energy density, ESLG) of the amorphous silicon thin film. When the energy density is lower than the nearly-completely-melted energy density, the seeds are not able to grow as large grains due to the insufficient energy provided to the seeds. The grains formed are thus small. When the energy density is higher than an amorphousization energy density (Eα) of the amorphous silicon thin film 12, the amorphous silicon thin film 12 is completely melted and then re-crystallize due to quench. The phenomenon of homogeneous nucleation therefore occurs to generate nucleation sites everywhere. Grains are not able to grow effectively so that the formed grain size is abruptly decreased, or is even amorphourized. When the energy density is between the super lateral growth energy density and the amorphousization energy density of the amorphous silicon thin film 12, small grains are simultaneously formed when large grains are formed. As a result, the grain size uniformity between devices cannot be well controlled to incur discrepancy of electrical performance between devices.
However, the prior art method of laser crystallization faces limitation. Please refer to FIG. 3A and FIG. 3B. FIG. 3A and FIG. 3B are cross-sectional diagrams illustrating amorphous silicon islands 20A, 20B after performing the prior art laser crystallization process. Since a photo-etching-process (PEP, not shown) is usually performed after forming the amorphous silicon thin film 12, in practical operation, to etch the amorphous silicon thin film 12 to amorphous silicon islands 20A, 20B, shown in FIG. 3A and FIG. 3B respectively. The amorphous silicon islands 20A, 20B may have various shapes according to the requirement of design and process. In FIG. 3A and FIG. 3B, an active area of a low temperature polysilicon thin film transistor (not shown), which is frequently seen, is adapted for illustrating.
As shown in FIG. 3A and FIG. 3B, thermal conduction rates of edge portions 22A, 22B of the amorphous silicon islands 20A, 20B are greater than thermal conduction rates of center portions 24A, 24B of the amorphous silicon island 20A, 20B to form temperature gradients due to unequal numbers of heat dissipating directions.
Therefore the amorphous silicon thin film in the edge portions 22A, 22B of the amorphous silicon islands 20A, 20B start to solidify after the amorphous silicon thin film in the edge portions 22A, 22B of the amorphous silicon islands 20A, 20B are nearly completely melted. Residual amorphous silicon seeds (not shown) in the amorphous silicon thin film in the edge portions 22A, 22B of the amorphous silicon islands 20A, 20B then grow laterally toward the center portions 24A, 24B of the amorphous silicon islands 20A, 20B to become large grains 26A, 26B.
However, the lateral growth rate has a specific limitation so that the final grain size is usually 1˜2 μm. As shown in FIG. 3A, the large grains 26A can reach to the center of the channel width to improve the electrical performance of device when the channel width is small. Therefore, the prior art method of laser crystallization the amorphous silicon island sometimes adapt the energy density higher than ESLG to enhance the driving ability of lateral growth. As shown in FIG. 3B, the large grains 26B are only formed in the edge portion 22B of the amorphous silicon islands 20B when the channel width is large. Many small grains 28 are formed in the center portion 24B of the amorphous silicon islands 20B to degrade the electrical performance of devices.
Furthermore, the range of the energy density adapted in the prior art laser crystallization process is too small. The laser energy density readily exceeds the above-mentioned range when the laser energy density has a small deviation. In addition, when other variables, such as the spatial uniformity of the laser energy, the overlapping ratio of the laser pulse, the substrate temperature during the laser annealing process and the atmosphere, are not controlled properly, the laser energy density readily exceeds the above-mentioned range too.
Therefore, it is very important to develop a method of laser crystallization so that the lateral growth of different portions of the amorphous silicon island is improved effectively to form homogeneous large grains and to enlarge the process window of the laser crystallization process.